Studies of the mechanisms and control of cellular reproduction are of central importance of basic biology and should also improve our understanding of clinical problems, such as developmental defects and cancer, that involve abnormal cellular reproduction. Because the eukaryotic cell-division cycle is a complex and poorly understood process, its successful analysis is unlikely unless the power of formal and molecular genetic methods can be brought to bear. For this and other reasons, the yeast Saccharomyces cerevisiae is an attractive candidate for cell-cyle studies. The yeast cell cycle has already been the subject of an intensive genetic analysis employing conventional temperature-sensitive (ts) mutants. This analysis has identified many genes whose products seem to function in specific steps of the cell cycle (CDC genes) and has been informative in various other ways, but suffers from several serious limitations, including (1) the limited usefulness of ts mutants in identifying gene products, (2) the limited usefulness of a set of ts mutants in exploring the functional organization of the cell cycle, and (3) the apparent impossibility of identifying more then a minority of CDC genes of means of ts mutations. The last problem illustrates a general mystery about cellular and genomic organization, namely the apparent discrepancy between genetic and molecular estimates of the number of genes. It should be possible to circumvent these limitations by pursuing other approaches to the identification of CDC genes. Thus, we now propose to isolate and analyze large sets of cold-sensitive (cs) cdc mutants, nonsense cdc mutants (using either temperature sensitive nonsense suppressors or phenotypic suppression by drugs), and apparent revertants of available ts or cs cdc mutants (in the expectation of identifying new genes of interest as extragenic suppressors of the original mutations). The mutants obtained will be analyzed geneticaly to determine which define new genes; the resulting statistics whould clarify the reasons for the discrepancy noted above. The new mutants will then be analyzed physiologically and morphologically (including electron microscopically) for clues as to the functions affected. The cs mutants and pseudorevertants should help reveal patterns of functional organization, and the nonsense mutants should greatly facilitate the identification of gene products in subsequent studies by us and others.